As LTE technology is being gradually developed, operators would like to reduce the cost of overall network maintenance by minimizing the number of different radio access technologies. Machine-Type Communications (MTC) devices tend to be low-end (i.e. low cost or low data rate) applications that could be handled adequately by Global System for Mobile Communication (GSM) or General Packet Radio Service (GPRS). As more and more MTC devices are deployed in the field, network operators may rely on existing GSM/GPRS networks for these low-end MTC devices. By adopting these low-cost MTC GSM/GPRS devices and with existing GSM/GPRS network, operators would be able to reap the maximum benefits out of their spectrum. In cellular systems, essential information about the cellular network are often carried in broadcast channels, and communicating data over broadcast channels that have sufficient coverage would be an indispensable element of the cellular system.
For example, when a user equipment (UE) initiate a cell search procedure in order to attach to a cell, the UE would be required to obtain information related to the cell identity and frame timing from a broadcast channel such as a physical broadcast channel (PBCH) in the case of a Long Term Evolution (LTE) communication system. If the UE is unable to discern information from the PBCH broadcasted from a cell, the UE would not able to attach to the cell. However, MTC UEs might be located in hard to reach places such as underground. Therefore, in order to minimize costs associated with additional radio access technologies, existing communication infrastructures may need to improve their coverage by an additional 15˜20 decibel (dB).
Such scenario could be illustrated in FIG. 1 in which a base station 101 or an evolved node B (eNB) in the case of LTE is shown to transmit or receive information from a MTC device 102 underneath a house. Without properly discerning the PBCH, the MTC device 102 would be unable to receive needed information in order to attach to the base station 101. Currently, the PBCH would include essential system information transmitted through the master information block (MIB).
FIG. 2 illustrates the content of MIB transmitted through the PBCH. The MIB would include the downlink bandwidth information represented by 3 bits, physical hybrid ARQ indicator channel (PHICH) configuration represented by 3 bits, the system frame number represented by 8 bits. The MIB content would further include 10 bits of reserved bits as well as 16 bits of cyclic redundancy check (CRC) for error detection. The MIB content channel will pass through a channel coding and modulation process before transmitted in the PBCH.
FIG. 3 illustrates a typical channel coding and modulation process. As shown in FIG. 3, a whole 40-bit content would pass through a channel coding process, a rate matching process, and a scrambling process. In the case of LTE implementation of the MIB in the PBCH transmission, the channel coding process would apply a tail-biting convolutional coding with ⅓ code rate. The rate matching process would then repeats 16 times for a robust transmission, and the scrambling process would be included to mitigate the inter-cell interference. Afterwards, the data would be modulated by QPSK for a low error rate. After modulation, processes involving layer mapping, precoding, resource mapping, and OFDM generation would be applied.
FIG. 4 illustrates a typical PBCH transmission. The content of MIB would typically repetitive and transmitted over 40 transmission time intervals (TTI) or 4 radio frames, and each radio frame would carry a segment of the MIB. In each radio frame, a segment of the MIB content would be carried by 6 resource blocks as shown in FIG. 4.
One of the objectives of LTE release 12 for low-cost MTC is to ensure that the service coverage would not be worse than GSM/GPRS so that the service would be at least comparable or may preferably be improved beyond what is possible in order to provide MTC services over GPRS/GSM as of today. A 15 dB improvement in coverage in comparison to defined LTE cell coverage footprint engineered for “normal LTE users (UEs)” should be targeted for low-cost MTC UEs, using very low rate traffic with relaxed latency. Thus, increasing the coverage has been the main concern for release 12.
In order to extend the PBCH coverage, one idea could be to increase repetition of the number of MIB transmissions. FIG. 5 illustrates an example of such concept by repeating a segment of MIB content multiple times within each radio frame. In each of the repetitions, a segment of MIB content is repeated identically. In this way, the system coverage of a cell could be extended at the expense of a large amount of system capacity.